(1) Field of the Invention
The present invention relates to liquid crystal displays and, more particularly, to a liquid crystal display having preferable viewing angle characteristics which has less variation in the contrast ratio, the luminance of transmitted light and tone attributable to changes in the viewing angle.
(2) Description of the Prior Art
Liquid crystal displays are widely used as displays capable of high definition color display for notebook type computers and display monitors.
Conventional liquid crystal displays include the so-called vertical field type liquid crystal displays utilizing a liquid crystal panel having a group of electrodes for pixel selection formed on each of a pair of upper and lower substrates represented by the twisted nematic (TN) type (generally referred to as "TN type liquid crystal displays") and the so-called horizontal field type liquid crystal displays utilizing a liquid crystal panel having a group of electrodes for pixel selection formed only on one of a pair of upper and lower substrates (generally referred to as "IPS type liquid crystal displays").
In the case of a liquid crystal panel forming a part of the former, i.e., the TN type liquid crystal display, liquid crystal is oriented at a twist of 90.degree. within a pair of (two) substrates, and two polarizing plates which are in the crossed Nicols configuration in the direction of the absorption axes and whose absorption axes at the entrance side are in parallel with or orthogonal to the rubbing direction are formed on the outer surface of the upper and lower substrates of the liquid crystal panel. When the rubbing direction and the absorption axes of the polarizing plates are in parallel with each other, such a state is referred to as "O" mode and, when they are perpendicular, the state is referred to as "E" mode.
In such a TN type liquid crystal display, incident light is transformed into linearly polarized light by the polarizing plate at the entrance side when no voltage is applied; the linearly polarized light propagates along the twist of the liquid crystal layer; the linearly polarized light is entirely emitted to show white display if the transmission axis of the polarizing plate at the exit side coincides with the azimuth angle of the linearly polarized light (the so-called normally open configuration).
When a voltage is applied, the direction of the unit vector (director) indicating an average direction of aligenment of the molecular axes of liquid crystal forming the liquid crystal layer is perpendicular to the surface of the substrates. There is no change in the azimuth angle of the linearly polarized light at the entrance side, and it therefore coincides with the absorption angle of the polarizing plate at the exit side to show black display (see "Principles and Applications of Liquid Crystal" issued by Institute of Industrial Research in 1991).
In an IPS type liquid crystal display in which a group of electrodes for pixel selection are formed on only one of a pair of substrates and a voltage is applied between adjoining electrodes on the substrate (between a pixel electrode and a counter electrode) to switch the liquid crystal layer in the direction parallel with the surface of the substrates, polarizing plates are provided such that black display appears when no voltage is applied (the so-called normally closed configuration).
The liquid crystal layer in such an IPS liquid crystal display is in homogeneous orientation in which it is in parallel with the substrate surface in the initial state and, in a plane parallel with the substrate, the director of the liquid crystal layer is in parallel with or at some angle to the direction in which the electrodes are provided when no voltage is applied. When a voltage is applied, the direction of the director of the liquid crystal layer changes to a direction perpendicular to the direction in which the electrodes are provided as the voltage is applied. When the direction of the director in the liquid crystal layer is inclined at 45.degree. toward the direction in which the electrodes are provided compared to the direction of the director with no voltage applied thereto, the liquid crystal layer during the application of a voltage causes the azimuth angle of polarized light to rotate at 90.degree. as if it is a 1/2 wavelength plate, which causes the transmission axis of the exit side polarizing plate and the azimuth angle of the polarized light to coincide thereby resulting in white display.
The IPS type liquid crystal display is characterized in that it is subjected to less variation in hue and contrast depending on the field angle and therefore allows wider field angles (see Japanese unexamined patent publication No. H5-505247).